Advanced, easy to use, asynchronous-capable DNS client
library and utilities.
adns is a resolver library for C (and C++) programs, and a collection
of useful DNS resolver utilities.
In contrast with the standard interfaces, gethostbyname et al and
libresolv, it has the following features:
- It is reasonably easy to use for simple programs which just want
to translate names to addresses, look up MX records, etc.
- It can be used in an asynchronous, non-blocking, manner. Many
queries can be handled simultaneously.
- Responses are decoded automatically into a natural representation
for a C program - there is no need to deal with DNS packet and RRDATA
- Sanity checking (eg, name syntax checking, reverse/forward
correspondence, CNAME pointing to CNAME) is performed automatically
- Time-to-live, CNAME and other similar information is returned in
an easy-to-use form, without getting in the way.
- There is no global state in the library; resolver state is an
opaque data structure which the client creates explicitly. A program
can have several instances of the resolver.
- Errors are reported to the application in a way that distinguishes
the various causes of failure properly.
- adns understands conventional resolv.conf, but this can overridden
by environment variables.
- Flexibility. For example, the application can tell adns to:
ignore environment variables (for setuid programs), disable hostname
syntax sanity checks to return arbitrary data, override or ignore
resolv.conf in favour of supplied configuration, etc.
- Believed to be correct ! For example, will correctly back off to
TCP in case of long replies or queries, or to other nameservers if
several are available. It has sensible handling of bad responses etc.
DNS utility programs
adns also comes with a number of utility programs for use from the
command line and in scripts:
adnslogres is a much faster version of Apache's
adnsresfilter is a filter which copies its input to
its output, replacing IP addresses by the corresponding names, without
unduly delaying the output. For example, you can usefully pipe the
output of netstat -n, tcpdump -ln, and the like, into it.
adnshost is a general-purpose DNS lookup utility
which can be used easily in from the command line and from shell
scripts to do simple lookups. In a more advanced mode it can be used
as a general-purpose DNS helper program for scripting languages which
can invoke and communicate with subprocesses. See the
usage message for a summary of its capabilities.
I'm afraid there is no manual yet. However, competent C programmers
should be able to use the library based on the
adns.h header file, and the usage messages for the programs should
I'd be pleased if you would let me know if you're using my library in
your project, and what you think of it.
Bug reports should be reported to the
GNU Debbugs. Send an email
email@example.com and at the top of your email,
in a paragraph of its own, write the single line
Your bug report will be published via to the adns-discuss list.
Feedback and discussion takes place on the
list. You can mail me privately
I have set up mailinglists
adns-discuss. The announcements list is moderated and
will contain only announcements of important bugs, new versions, etc.
and subscription web pages, or you can subscribe by sending mail
containing the word `subscribe' to
Download and source code
adns is also available from the
GNU Project FTP servers and their
adns requires a real nameserver like BIND running on the same system
or a nearby one, which must be willing to provide `recursive service'.
I.e., adns is a `stub resolver'.
adns requires that your real nameserver is on the same machine, or
connected via a secure network, so that an attacker cannot fake the
replies to adns's queries.
References and related projects
Copyright and licensing
adns is Copyright 1997-2000,2003,2006,2014 Ian Jackson,
Copyright 2014 Mark Wooding, Copyright 1999-2000,2003,2006 Tony Finch,
and Copyright (C) 1991 Massachusetts Institute of Technology.
adns is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or (at
your option) any later version.
This program and documentation is distributed in the hope that it will
be useful, but without any warranty; without even the implied
warranty of merchantability or fitness for a particular
purpose. See the
General Public License for more details.
You should have received a copy of the GNU General Public License
along with adns, or one should be available above; if not,
write to the Free Software Foundation
Ian Jackson / firstname.lastname@example.org.
GNU home page;
chiark home page;
site or mirror home page
This web page is Copyright (C)1996-2005,2014 Ian Jackson. See the